HP to Amps Calculator

For a 3-phase motor: Amps = (HP x 745.7) / (V x eta x PF x 1.732). Where eta is motor efficiency as a decimal (e.g., 0.93 for 93%) and PF is power factor (e.g., 0.88). For field use and NEC-compliant design, Table 430.250 provides pre-calculated full-load currents that are the code-required basis for conductor and breaker sizing in North America. The calculated value using actual nameplate efficiency and PF will typically be 10-20% lower than the conservative NEC table value, reflecting the improved performance of modern NEMA Premium motors.

At 480V 3-phase per NEC Table 430.250: 14 A. At 208V 3-phase: 30.8 A. At 230V single-phase: 50 A per NEC Table 430.248. The current varies dramatically with voltage -- always check the motor nameplate and the appropriate NEC table for the specific voltage and phase configuration. For energy analysis, a modern 10 HP NEMA Premium motor at 460V 3-phase with 91.7% efficiency and 0.85 PF draws approximately 13.3 A -- close to but slightly under the NEC table value of 14 A. Note that a 10 HP motor operating on a 208V system draws more than double the current at 480V, which means double the conductor cost and I^2R losses for the same mechanical output.

NEC Table 430.250 uses conservative assumptions for motor efficiency and power factor to ensure conductors and overcurrent protection are adequately sized even for older, less efficient motors. The NEC values provide a safe, standardized basis for electrical design that works for any motor meeting NEMA MG 1 standards. Your nameplate FLA or calculated amps may be lower, but the NEC table FLA is the mandatory minimum for conductor sizing (NEC 430.22) and overcurrent protection maximum settings (NEC 430.52). The 15-20% margin also accommodates future motor replacement: if a facility replaces a 2025 NEMA Premium motor (95.4% eff) with a standard-efficiency replacement in 2040 (88% eff), the 15-year-old conductors and breakers sized from the NEC table will still be adequate because the table values anticipated that possibility.

Motor efficiency has a direct inverse relationship with current: I = HP x 745.7 / (V x eta x PF x sqrt(3)). A 100 HP motor at 88% efficiency draws 74,570/(480 x 0.88 x 0.85 x 1.732) = 120.0 A. The same motor with 95.4% NEMA Premium efficiency draws 74,570/(480 x 0.954 x 0.88 x 1.732) = 107.3 A -- a 12.7 A reduction (10.6%). This efficiency improvement has compounding financial benefits: lower conductor losses (I^2R), reduced voltage drop, smaller conductors possible, lower air conditioning load in electrical rooms, and avoided utility demand charges. Over a 20-year motor life at 8,000 hours/year and $0.10/kWh, the efficiency difference saves approximately 12.7 x 480 x 1.732 x 0.88 / 1000 x 8000 x 20 x $0.10 = $148,000 in energy costs -- far exceeding the incremental cost of the premium motor.

For the same horsepower and voltage, a single-phase motor draws approximately 1.732 times (73% more) current than a three-phase motor because the single-phase formula omits the sqrt(3) factor in the denominator. A 5 HP 230V three-phase motor draws 15.2 A (NEC Table 430.250), while a 5 HP 230V single-phase motor draws 28 A (NEC Table 430.248) -- nearly double. This higher current requires larger conductors, larger contactors, and incurs greater I^2R losses, which is why single-phase motors above 10 HP are rare in industrial settings. For applications where only single-phase power is available, the HP-to-amps calculation reveals the practical limit: a 15 HP single-phase motor at 230V draws approximately 84 A (NEC table), requiring a 125 A breaker and #2 AWG copper conductors -- the practical maximum for a residential or light commercial 200 A single-phase service.

NEMA motors typically have a service factor (SF) of 1.15, meaning they can continuously deliver 115% of rated HP without overheating. At service factor load, the current increases approximately proportionally: a 100 HP motor drawing 124 A (NEC table) at full load would draw approximately 124 x 1.15 = 143 A at 115 HP. However, DO NOT use the service factor current for conductor sizing -- NEC 430.22 requires conductors sized at 125% of the full-load (100% HP) current from the NEC table. For overload protection, NEC 430.32(C) allows the overload setting to be based on nameplate FLA x SF (for SF >= 1.15), providing operational headroom without nuisance trips. Routine operation at service factor load is not recommended: motor insulation life decreases by approximately 50% for every 10C increase in winding temperature, and operating in the service factor typically adds 10-15C to the winding temperature rise.